1. Field of the Invention
This invention relates to loudspeakers and more particularly to loudspeakers having a flat panel design.
2. Description of the Related Art
Dynamic loudspeakers typically include a relatively stiff diaphragm that is coupled to an electromagnetic driver assembly, which basically comprises a voice coil and a permanent magnet. Such loudspeakers are usually mounted so as to occupy an opening in an enclosure or baffle. The interaction of the magnetic field of the permanent magnet and the varying magnetic field of the voice coil that is produced when a changing current is passed through the voice coil causes the loudspeaker diaphragm to vibrate. Vibration of the diaphragm causes movement of air, which in turn produces sound.
The advantages of the moving-coil drive unit are that its operation and design are widely understood and used, the components parts are readily available and it is inexpensive to produce. One disadvantage is that this drive unit is very inefficient as a transducer, typically converting between 1 and 3% of the electrical energy into sound energy. Another disadvantage of moving-coil drive units is that the mechanical inertia resulting from the mass of the driver itself makes it impossible for the driving part to start and stop instantly. This sets a limit on the transducer's bandwidth and on its ability to reproduce transients clearly.
To overcome the disadvantages of the typical moving-coil drive units, there has been developments in the areas of “mass-less” drivers. One such driver is the piezoelectric type. A piezoelectric speaker utilizes crystalline materials that will twist or bend mechanically when a voltage is applied. The resulting movement is very small and in practice crystal transducers are generally matched to a horn to improve efficiency. The problem with the piezoelectric transducer is that it has a limited bandwidth and its application is therefore limited to reasonably flat frequency response and low coloration.
Another attempt at the “mass-less” drive unit has been the flat panel loudspeaker, which uses low mass sheets or film in place of a cone diaphragm. The operating principle of the traditional electrostatic flat speaker is that of a two plate capacitor. One plate is a fixed electrode, the other is a stretched conductive plastic film. Both the audio signal and a DC polarizing voltage are applied across the plates. The applied voltage is varied in accordance with the audio signal. The charge between the plates also varies. The size of the electrostatic charge determines the attractive force and thus the film diaphragm is set in motion.
The loudness of the sound produced by a loudspeaker is related to the volume of air moved in from the loudspeaker by vibration of the diaphragm. Generally, the greater the volume of air moved by the diaphragm as it vibrates, the greater the loudness. The loudness of sound produced relative to the electrical energy provided as an electric current through the voice coil is also used to measure the efficiency of the loudspeaker.
It is desirous to make speakers more compact and flat for easy installation in locations with restricted areas such as walls, panels and other flat surface areas. The disadvantage of the electrostatic flat speaker is that manufacturing is difficult. This speaker requires a DC voltage source and a step-up transformer for impedance matching, which creates additional expense. Also, the speaker would have to be large to create good bass.
Even the smallest conventional speakers that use relatively rigid paper or plastic cones, or diaphragms, require an air enclosure having a thickness dimension typically well in excess of three inches. This is ordinarily required to provide acceptable sound reproduction in the low/mid frequency regions where voices and musical instruments produce most of their sound energy. The air enclosures, however, inherently “resonate” in such a manner as to accentuate some frequencies while diminishing others, thereby significantly detracting from the naturalness and clarity of the reproduced sound. It is desirable to have a speaker without the air enclosure, thus without the altered and unnatural acoustic effect, and with improved sound quality and a reduction in speaker thickness.
Additionally, high quality conventional cone speakers inherently require multiple speaker elements, known as woofers, midranges, and tweeters, each specializing in the reproduction of a different frequency range of sound. The difficulty with such multi-element designs is that the transitions between the speaker elements cannot be smoothly blended at all listening angles, which again results in reduced naturalness and clarity of the reproduced sound.
A known flat panel loudspeaker has been developed which uses a very stiff panel whose characteristics must conform to a specific mathematical relationship. This panel can be excited by a transducer such as a moving-coil element or a piezoelectric crystal. If all the parameters are met, the panel has a complex bending behavior resulting in a large number of seemingly randomized vibrational modes distributed across the panel surface. The disadvantage of this device is that the complex bending behavior of the panel requires precise manufacturing, which is costly and time consuming.
It is, therefore, desirable to have a compact, flat speaker with a non-rigid planar diaphragm that emits high quality sound over a wide bandwidth while maintaining low manufacturing costs.